The recovery of gold and silver from ores and concentrates has remained largely unaltered for over a century, in that alkaline cyanide solutions have been universally used for their dissolution. Various hydrometallurgical and pyrometallurgical techniques, such as roasting, pressure leaching and alkaline chlorination have been developed, as pre-treatment processes prior to cyanidation, for their recovery from the so-called refractory and carbonaceous feedstocks, where the gold and silver cannot be recovered directly by cyanidation. None of these processes has replaced cyanide as the leaching medium for gold and silver. This is evident from an article by J. O. Marsden entitled Overview of Gold Processing Techniques Around the World, published in Minerals and Metallurgical Processing, August 2006, volume 23, number 3.
Refractory gold ores are ones that are naturally resistant to cyanide leaching. These include, but are not limited to, arsenopyrite ores, where the gold is intimately locked up in the arsenopyrite matrix. In order to liberate the gold, this matrix must be destroyed, which also liberates the arsenic, and hence a separate additional process is required in order to fix the arsenic in an environmentally-acceptable manner. Destruction of the arsenopyrite, and similar gold-bearing matrices, is commonly achieved by pressure oxidation in an autoclave at around 220° C., or by roasting of the ore. Roasting generates an arsenical off-gas also containing sulfur dioxide, which has to be treated for the collection of both of these compounds.
Carbonaceous ores are often amenable to leaching, but once the gold is taken into solution, it is then re-adsorbed by the active carbon in the ore thus making it unrecoverable. Rendering of the carbon passive is also accomplished by roasting, and also by alkaline chlorination (hypochlorite), but this generates a chloride-containing effluent which has to be contained in a dedicated impoundment area.
A number of different leaching agents have been tested without any commercial success in an attempt to replace cyanide for environmental reasons. These compounds include, for instance, thiourea and thiosulfate, which are much more effective for silver than for gold. Thiourea, however, suffers from the same drawbacks as cyanide in that it is toxic and a listed carcinogen.
Wilson in U.S. Pat. No. 4,551,213 Recovery of Gold, issued on Nov. 5, 1985, and the accompanying article An Economical Method for the Recovery of Gold from the Sulphur Containing Residue of a Hydrometallurgical Process in Complex Sulfides, Processing of Ores, Concentrates and By-Products (A. D. Zunkel, R. S. Boorman, A. E. Morris and R. J. Wesely, Editors), published by TMS in 1985, developed a process for the recovery of gold from their CLEAR (Copper Leaching, Electrowinning and Recycle) Process residues using highly concentrated cupric chloride.
The Wilson process uses a high oxidation potential (650-750 millivolts) of cupric and/or ferric chloride in concentrated solutions (12 weight percent of chloride) at temperatures up to 106° C., the boiling point of such solutions. The gold is recovered directly by electrolysis or by adsorption onto activated carbon.
Such a process is capable of dissolving gold from a variety of feed materials, and is effective with some, but not all refractory-type gold ores, since the patent notes that pyrite remains in the residue. Pyrite is a noted host of gold values. It is also not effective with carbonaceous ores, where the active carbon in the ore adsorbs any gold dissolved, as evidenced by the fact that active carbon is one of the chosen methods for recovery of gold from the solution.
Intec Limited recently published on their web site www.intec.com.au in March, 2009 a halide-based process, The Intec Gold Process. This process makes us of the unique properties of Halex™, a composite chloride-bromide ion with a high oxidation potential. The Intec Process is operated at temperatures below 95° C. in 6-8M chloride solutions. Leaching time can be up to 10 hours.
Higher temperatures cannot be employed in the Intec Process because it would render the Halex™ molecule unstable. The Intec publication notes that carbonaceous ores cannot be treated by this process, the reasons being the same as for the Wilson process above.
The use of halides, particularly chloride, has been employed in pressure leaching situations to dissolve gold and precious metals, as described by Christopher A. Fleming et al., in U.S. Pat. No. 6,315,812 Oxidative Pressure Leach Recovery Using Halide Ions. In this process, halide ions, especially chloride ions (preferably in the range up to 10 g/L), are added to a pressure leaching autoclave at temperatures >200° C., wherein the gold and platinum group metals are dissolved into solution. They are subsequently recovered by one of a variety of methods.
Yoshifumo Abe, et al., in US Patent Application US 2009/0241735 A1, dated Oct. 1, 2009 for recovering gold from copper sulfide ores. The process uses both ferric and cupric chloride, but no acid is added, and works better if bromide ions are also present, in accordance with the process of Intec described above. The examples in the application indicate that extraction from ores containing less than 10 g/t gold would not proceed, and therefore that the process would not work on typical gold ores which usually contain gold in the concentration range 1-5 g/t.
No indication is given of how the spent leaching solution would be treated, since it is clear that the process also dissolves iron. The process would be prohibitively expensive without recycle of the chloride ions, and there is no obvious method how this might be achieved. The process as described, therefore, would seem to be wholly unsuited to refractory gold ores.
Another method which has been developed especially for silver is nitrogen species catalysis (known as NSC) in alkaline and acid sulfate media. One article describing these processes is by Corby G. Anderson, Treatment of Copper Ores and Concentrates with Industrial Nitrogen Species Catalyzed Pressure Leaching and Non-Cyanide Precious Metals Recovery in Journal of Metals, April 2003. In these processes, silver and other metals are dissolved into solution using NSC to effect oxidation. Gold remains in the leaching residue, and is subsequently recovered by either conventional cyanidation or by concentrated caustic leaching, causing it to form complex polysulfides.
Rein Raudsepp and Morris J. Beattie, as described in U.S. Pat. No. 4,878,945, Hydrometallurgical Process for Treating Refractory Ores Containing Precious Metals, dated Nov. 7, 1989, also used nitrogen species to break down pyrite and arsenopyrite in refractory ores. In this process, however, gold was not dissolved, and silver only partially, requiring a separate cyanidation or other recovery step. The re-constitution of the nitrogen species was achieved with oxygen in a pressure vessel in a separate leaching step.
NSC has not been used in chloride systems, apart from its use as aqua regia in precious metal refineries, where no attempt is made to collect and re-use the NOx fumes which result from its use. Aqua Regia, however, is one part nitric acid and three parts hydrochloric acid, and has to be mixed in a special way, hence there is a very large concentration of nitric species present, as well as unique NOCl species.
A further impediment to developing any new extractive recovery processes has been the dissolution and subsequent control of iron, which is and has always been considered a major problem in hydrometallurgical processes. In atmospheric processes, the iron is usually eliminated from the process solutions by precipitation with a base such as lime, magnesia or caustic soda, as an oxy-hydroxide, and in higher temperature autoclave processes, as an impure hematite or goethite.
It is evident from the foregoing that although there are techniques available for dealing with the various types of gold ores, there is no one universal method for all types.